Battery thermal management system

ABSTRACT

A battery thermal management system according to an exemplary aspect of the present disclosure includes, among other things, a battery assembly and a coolant subsystem that circulates coolant through the battery assembly. The battery assembly is heated by a first portion of the coolant from an engine if a temperature of the battery assembly is below a first temperature threshold and is cooled by a second portion of the coolant from a chiller if the temperature is above a second temperature threshold.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional of U.S. patent application Ser. No. 14/630,951,which was filed on Feb. 25, 2015, the entire disclosure of which ishereby incorporated by reference.

TECHNICAL FIELD

This disclosure relates to a battery thermal management system for anelectrified vehicle. The battery thermal management system is configuredto heat a battery assembly if its temperature is below a firsttemperature threshold and cool the battery assembly if its temperatureis above a second temperature threshold.

BACKGROUND

The need to reduce fuel consumption and emissions in vehicles is wellknown. Therefore, vehicles are being developed that reduce or completelyeliminate reliance on internal combustion engines. Electrified vehiclesare one type of vehicle being developed for this purpose. In general,electrified vehicles differ from conventional motor vehicles becausethey are selectively driven by one or more battery powered electricmachines. Conventional motor vehicles, by contrast, rely exclusively onthe internal combustion engine to propel the vehicle.

A high voltage battery pack typically powers the electric machines of anelectrified vehicle. The battery pack may include one or more groupingsof interconnected battery cells. The battery cells generate heat duringcertain conditions, such as charging and discharging operations. Batterythermal management systems are employed to manage the heat generated bythe battery cells of the battery pack.

SUMMARY

A battery thermal management system according to an exemplary aspect ofthe present disclosure includes, among other things, a battery assemblyand a coolant subsystem that circulates coolant through the batteryassembly. The battery assembly is heated by a first portion of thecoolant from an engine if a temperature of the battery assembly is belowa first temperature threshold and is cooled by a second portion of thecoolant from a chiller if the temperature is above a second temperaturethreshold.

In a further non-limiting embodiment of the foregoing system, thecoolant subsystem includes the engine, a radiator, a three-way valve, atemperature sensor, a T-joint, a pump, and a chiller loop that includesthe chiller.

In a further non-limiting embodiment of either of the foregoing systems,a refrigerant subsystem circulates a refrigerant, the refrigerantexchanging heat with the second portion of the coolant within thechiller.

In a further non-limiting embodiment of any of the foregoing systems,the coolant subsystem includes a three-way valve that controls the flowof the first portion and the second portion of the coolant to thebattery assembly.

In a further non-limiting embodiment of any of the foregoing systems,the three-way valve is positioned between the engine and the batteryassembly and between the chiller and the battery assembly.

In a further non-limiting embodiment of any of the foregoing systems, acontroller is configured to control communication of the first portionand the second portion of the coolant through the battery assembly.

In a further non-limiting embodiment of any of the foregoing systems,the controller is configured to open a first inlet of a three-way valveto deliver the first portion of the coolant to the battery assembly andis configured to open a second inlet of the three-way valve to deliverthe second portion of the coolant to the battery assembly.

In a further non-limiting embodiment of any of the foregoing systems,the coolant subsystem includes a radiator configured to cool the engine.

In a further non-limiting embodiment of any of the foregoing systems, aT-joint splits a flow of coolant exiting the battery assembly between achiller loop that includes the chiller and the engine.

In a further non-limiting embodiment of any of the foregoing systems,the engine includes a thermostat that controls flow of the coolantexiting the engine.

A method according to another exemplary aspect of the present disclosureincludes, among other things, heating a battery assembly using coolantfrom an engine if a temperature of the battery assembly is below a firsttemperature threshold and cooling the battery assembly using coolantfrom a chiller if the temperature is above a second temperaturethreshold.

In a further non-limiting embodiment of the foregoing method, the methodincludes performing heat transfer between the coolant from the chillerand a refrigerant.

In a further non-limiting embodiment of either of the foregoing methods,the method includes opening a first inlet of a three-way valve todeliver the coolant from the engine to the battery assembly if thetemperature is below the first temperature threshold.

In a further non-limiting embodiment of any of the foregoing methods,the method includes opening a second inlet of the three-way valve todeliver the coolant from the chiller to the battery assembly if thetemperature is above the second temperature threshold.

In a further non-limiting embodiment of any of the foregoing methods,the method includes monitoring a temperature of the battery assemblyprior to the heating and cooling steps.

In a further non-limiting embodiment of any of the foregoing methods,the method includes dividing the flow of the coolant from the enginebetween a radiator and a three-way valve positioned upstream from thebattery assembly.

The embodiments, examples and alternatives of the preceding paragraphs,the claims, or the following description and drawings, including any oftheir various aspects or respective individual features, may be takenindependently or in any combination. Features described in connectionwith one embodiment are applicable to all embodiments, unless suchfeatures are incompatible.

The various features and advantages of this disclosure will becomeapparent to those skilled in the art from the following detaileddescription. The drawings that accompany the detailed description can bebriefly described as follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a powertrain of an electrified vehicle.

FIG. 2 illustrates a battery thermal management system for anelectrified vehicle.

FIG. 3 schematically illustrates an exemplary control strategy forthermally managing a battery assembly of an electrified vehicle.

DETAILED DESCRIPTION

This disclosure describes a battery thermal management system for anelectrified vehicle. The battery thermal management system includes acoolant subsystem that circulates coolant through the battery assemblyto either heat or cool the battery assembly. The coolant may exchangeheat with refrigerant of a refrigerant subsystem. In some embodiments,the battery assembly is heated by coolant communicated from an engine ifa temperature of the battery assembly is below a first temperaturethreshold. In other embodiments, the battery assembly is cooled bycoolant communicated from a chiller if the temperature of the batteryassembly is above a second temperature threshold. These and otherfeatures are discussed in greater detail in the following paragraphs ofthis detailed description.

FIG. 1 schematically illustrates a powertrain 10 for an electrifiedvehicle 12. Although depicted as a hybrid electric vehicle (HEV), itshould be understood that the concepts described herein are not limitedto HEV's and could extend to other electrified vehicles, including butnot limited to, plug-in hybrid electric vehicles (PHEV's).

In one non-limiting embodiment, the powertrain 10 is a power-splitpowertrain system that employs a first drive system and a second drivesystem. The first drive system includes a combination of an engine 14and a generator 18 (i.e., a first electric machine). The second drivesystem includes at least a motor 22 (i.e., a second electric machine)and a battery assembly 24. In this example, the second drive system isconsidered an electric drive system of the powertrain 10. The first andsecond drive systems generate torque to drive one or more sets ofvehicle drive wheels 28 of the electrified vehicle 12. Although apower-split configuration is shown, this disclosure extends to anyhybrid or electric vehicle including full hybrids, parallel hybrids,series hybrids, mild hybrids or micro hybrids.

The engine 14, which in one embodiment is an internal combustion engine,and the generator 18 may be connected through a power transfer unit 30,such as a planetary gear set. Of course, other types of power transferunits, including other gear sets and transmissions, may be used toconnect the engine 14 to the generator 18. In one non-limitingembodiment, the power transfer unit 30 is a planetary gear set thatincludes a ring gear 32, a sun gear 34, and a carrier assembly 36.

The generator 18 can be driven by the engine 14 through the powertransfer unit 30 to convert kinetic energy to electrical energy. Thegenerator 18 can alternatively function as a motor to convert electricalenergy into kinetic energy, thereby outputting torque to a shaft 38connected to the power transfer unit 30. Because the generator 18 isoperatively connected to the engine 14, the speed of the engine 14 canbe controlled by the generator 18.

The ring gear 32 of the power transfer unit 30 may be connected to ashaft 40, which is connected to vehicle drive wheels 28 through a secondpower transfer unit 44. The second power transfer unit 44 may include agear set having a plurality of gears 46. Other power transfer units mayalso be suitable. The gears 46 transfer torque from the engine 14 to adifferential 48 to ultimately provide traction to the vehicle drivewheels 28. The differential 48 may include a plurality of gears thatenable the transfer of torque to the vehicle drive wheels 28. In oneembodiment, the second power transfer unit 44 is mechanically coupled toan axle 50 through the differential 48 to distribute torque to thevehicle drive wheels 28.

The motor 22 can also be employed to drive the vehicle drive wheels 28by outputting torque to a shaft 52 that is also connected to the secondpower transfer unit 44. In one embodiment, the motor 22 and thegenerator 18 cooperate as part of a regenerative braking system in whichboth the motor 22 and the generator 18 can be employed as motors tooutput torque. For example, the motor 22 and the generator 18 can eachoutput electrical power to the battery assembly 24.

The battery assembly 24 is an example type of electrified vehiclebattery. The battery assembly 24 may include a high voltage tractionbattery pack that includes a plurality of battery cells capable ofoutputting electrical power to operate the motor 22 and the generator18. Other types of energy storage devices and/or output devices can alsobe used to electrically power the electrified vehicle 12.

In one non-limiting embodiment, the electrified vehicle 12 has two basicoperating modes. The electrified vehicle 12 may operate in an ElectricVehicle (EV) mode where the motor 22 is used (generally withoutassistance from the engine 14) for vehicle propulsion, thereby depletingthe battery assembly 24 state of charge up to its maximum allowabledischarging rate under certain driving patterns/cycles. The EV mode isan example of a charge depleting mode of operation for the electrifiedvehicle 12. During EV mode, the state of charge of the battery assembly24 may increase in some circumstances, for example due to a period ofregenerative braking. The engine 14 is generally OFF under a default EVmode but could be operated as necessary based on a vehicle system stateor as permitted by the operator.

The electrified vehicle 12 may additionally operate in a Hybrid (HEV)mode in which the engine 14 and the motor 22 are both used for vehiclepropulsion. The HEV mode is an example of a charge sustaining mode ofoperation for the electrified vehicle 12. During the HEV mode, theelectrified vehicle 12 may reduce the motor 22 propulsion usage in orderto maintain the state of charge of the battery assembly 24 at a constantor approximately constant level by increasing the engine 14 propulsionusage. The electrified vehicle 12 may be operated in other operatingmodes in addition to the EV and HEV modes within the scope of thisdisclosure.

FIG. 2 schematically illustrates a battery thermal management system 54that can be incorporated into an electrified vehicle, such as theelectrified vehicle 12 of FIG. 1. The battery thermal management system54 may be used to manage the thermal load generated by various vehiclecomponents, such as the engine 14 and the battery assembly 24. In oneembodiment, the battery thermal management system 54 selectivelycommunicates coolant to the battery assembly 24 to either heat or coolthe battery assembly 24 depending on its temperature and/or otherconditions. Although not shown, the battery assembly 24 may include aplurality of battery cells that supply electrical power to thecomponents of the vehicle. The battery assembly 24 may include one ormore groupings of battery cells, which are sometimes referred to as“battery arrays.”

The battery thermal management system 54 may include a coolant subsystem56 and a refrigerant subsystem 58. The coolant subsystem 56 is shown indashed lines and the refrigerant subsystem 58 is shown in solid lines.These systems are described in detail below.

The coolant subsystem 56, or coolant loop, may circulate a coolant C,such as glycol or any other coolant, to thermally manage the batteryassembly 24. In one embodiment, the coolant subsystem 56 includes theengine 14, a radiator 60, a three-way valve 62, a temperature sensor 64,a T-joint 66 and a pump 68. The coolant subsystem 56 may additionallyinclude a chiller loop 70 that includes a chiller 72 and a pump 74.Although not shown, the various components of the coolant subsystem 56can be fluidly interconnected by conduits or passages such as tubes,hoses, pipes, heater core, oil cooler, PTC heater, engine oil cooler,exhaust heat recovery system, etc.

In operation of the coolant subsystem 56, the pump 68, which may be anengine pump that is operatively coupled to the engine 14, communicatescoolant C to the engine 14. The coolant C picks up heat within theengine 14. A portion C1 of the coolant C may be communicated to theradiator 60. A fan 76 draws airflow F through the radiator 60 forundergoing heat transfer with the portion C1 of the coolant C. Forexample, heat from the portion C1 of the coolant C is dumped to theairflow F to cool the portion C1 of the coolant C. Cooled coolant C canthen be communicated back to the engine 14 for cooling the engine 14.

Meanwhile, another portion C2 of the coolant C may exit the engine 14 atthermostat 78 into line 75. In one embodiment, the thermostat 78 is adual stage continuous regulator valve configured to regulate the flow ofthe coolant C. Under certain operating conditions, the thermostat 78 mayprevent communication of the portions C1 and C2 of the coolant C.

The portion C2 of the coolant C may be communicated within line 75 to aninlet 80 of the three-way valve 62. The three-way valve 62 is positionedupstream from the battery assembly 24 to control the flow of the portionC2 of the coolant C through the battery assembly 24. During certainconditions, the inlet 80 of the three-way valve 62 may be actuated(i.e., opened) to communicate the portion C2 of the coolant C to warmthe battery assembly 24. In one non-limiting embodiment, the portion C2of the coolant C may be communicated through a passage 77 of the batteryassembly 24. The passage 77 may take any size, shape or configurationand is not limited to the schematic depiction of FIG. 2.

Alternatively, the chiller loop 70 may be used to deliver anotherportion C3 of the coolant C for cooling the battery assembly 24. CoolantC is cooled within the chiller 72 to provide the portion C3. The portionC3 of the coolant C is then communicated to an inlet 82, which isseparate from the inlet 80, of the three-way valve 62. The inlet 82 maybe selectively opened and the inlet 80 selectively closed to deliver theportion C3 of the coolant C to the battery assembly 24 for cooling. Thepump 74 may be positioned between the chiller 72 and the three-way valve62 to circulate the portion C3 of the coolant C as necessary.

The temperature sensor 64 is positioned between the battery assembly 24and the T-joint 66. Alternatively, the temperature sensor 64 could bepositioned upstream from or inside of the battery assembly 24. Thetemperature sensor 64 may be used to detect a temperature of the coolantC that exits the battery assembly 24.

The T-joint 66 may be located downstream from the battery assembly 24.The T-joint 66 is adapted to split the coolant C that exits the batteryassembly between the chiller loop 70 and line 84. Coolant C that entersthe chiller loop 70 is communicated to the chiller 72 for exchangingheat with refrigerant R of the refrigerant subsystem 58 to generate thecooled portion C3 of the coolant C. In other words, the chiller 72facilitates the transfer of thermal energy between the chiller loop 70and the refrigerant subsystem 58. When the chiller loop 70 is notrunning, coolant C that enters line 84 is returned to engine 14 tocomplete the closed loop coolant subsystem 56.

The refrigerant subsystem 58, or refrigerant loop, may circulaterefrigerant R to transfer thermal energy to or from a passenger cabin 86and/or to or from the chiller loop 70. For example, the refrigerantsubsystem 58 may be part of a main vehicle cooling system that isconfigured to deliver conditioned airflow to the passenger cabin 86. Therefrigerant subsystem 58 is also configured to exchange heat with thechiller loop 70 via the chiller 72, as further discussed below. Althoughnot shown, the various components of the refrigerant subsystem 58 can befluidly interconnected by conduits or passages such as tubes, hoses,pipes, and/or the like.

In one non-limiting embodiment, the refrigerant subsystem 58 includes acompressor 88, a condenser 90 and an evaporator 92. The chiller 72 ofthe chiller loop 70 is also in fluid communication with the refrigerantsubsystem 58.

During operation of the refrigerant subsystem 58, the compressor 88pressurizes and circulates refrigerant through the refrigerant subsystem58. The compressor 88 may be powered by an electrical or non-electricalpower source. For example, the compressor 88 could be operativelycoupled to the engine 14 or driven by an electrically powered motor. Thecompressor 88 directs high pressure refrigerant R to the condenser 90.

The high pressure refrigerant R may next exchange heat with the airflowF from the fan 76 within the condenser 90. The condenser 90 transfersheat to the surrounding environment by condensing the refrigerant R froma vapor to a liquid.

The liquid refrigerant R exiting the condenser 90 may be communicated toa receiver dryer. The receiver dryer 94 separates entrained air andgases in the refrigerant R as it flows through the receiver dryer 94.

The refrigerant R is next communicated to a T-joint 96. In oneembodiment, the T-joint 96 splits the flow of the refrigerant R betweenportions R1, R2. The portion R1 of the refrigerant R is communicated toan on/off switch 98 that is configured to control the flow of theportion R1 of the refrigerant R to the evaporator 92. The on/off switch98 may or may not be externally controlled.

Within the evaporator 92, heat is transferred between the surroundingenvironment and the portion R1 of the refrigerant R, thereby causing theportion R1 of the refrigerant R to vaporize. A fan 99 may communicate anairflow F across the evaporator 92 for effectuating such heat transferand to deliver conditioned airflow to the passenger cabin 86 ascommanded by a driver operator.

A second portion R2 of the refrigerant R may be communicated from theT-joint 96 to another on/off switch 100 in line 101. The on/off switch100 is located between the T-joint 96 and the chiller 72 of the chillerloop 70 and is configured to control the flow of the portion R2 of therefrigerant R similar to the on/off switch 98.

The chiller 72 of the chiller loop 70 is in fluid communication with therefrigerant subsystem 58. In this way, the chiller 72 is part of boththe chiller loop 70 and the refrigerant subsystem 58. The portion R2 ofthe refrigerant R may exchange heat with the coolant C of the chillerloop 70 within the chiller 72 to provide the cooled portion C3 of thecoolant C, which can be used to cool the battery assembly 24.Refrigerant R exiting the chiller 72 is delivered to another T-joint102. The refrigerant R exiting the chiller 72 combines with refrigerantR exiting the evaporator 92 within the T-joint 102. The combinedrefrigerant R is returned to the compressor 88 and then back to thecondenser 90 as part of a closed loop system.

The battery thermal management system 54 may additionally include acontroller 104. The controller 104 is configured to control operation ofthe coolant subsystem 56 and the refrigerant subsystem 58 with thechiller loop 70 to either heat or cool the battery assembly 24. Thecontroller 104 may be part of an overall vehicle control unit, such as avehicle system controller (VSC), or could alternatively be a stand-alonecontrol unit separate from the VSC. In one embodiment, the controller104 includes executable instructions for interfacing with and operatingthe various components of the battery thermal management system 54. Thecontroller 104 may include inputs and outputs for interfacing with thevarious components of the battery thermal management system 54. Thecontroller 104 may additionally include a processing unit andnon-transitory memory for executing the various control strategies andmodes of the battery thermal management system 54.

In one embodiment, the controller 104 is adapted to monitor atemperature of the battery assembly 24. The controller 104 may receivefeedback from various sensors that monitor the temperature of thebattery assembly 24, including but not limited to ambient sensors andbattery cell sensors. Based on feedback from such sensors, thecontroller 104 can command opening or closing of the inlets 80, 82 ofthe three-way valve 62 to deliver either the portion C2 of the coolant Cfor warming the battery assembly 24 or the portion C3 of the coolant Cfor cooling the battery assembly 24.

FIG. 3, with continued reference to FIGS. 1 and 2, schematicallyillustrates a control strategy 200 for thermally managing the batteryassembly 24 of the electrified vehicle 12. For example, the controlstrategy 200 may be executed during certain conditions to either heat orcool the battery assembly 24 depending on the temperature of the batteryassembly 24, ambient conditions, any heat source near the batteryassembly 24, among other factors. Of course, the electrified vehicle 12is capable of implementing and executing other control strategies withinthe scope of this disclosure. In one embodiment, the controller 104 ofthe battery thermal management system 54 is programmed with one or morealgorithms adapted to execute the control strategy 200, or any othercontrol strategy. In other words, the control strategy 200 may be storedas executable instructions in the non-transitory memory of thecontroller 104. In another embodiment, the control strategy 200 isstored in the vehicle system controller (VSC), which can communicatewith the controller 104 to execute operation of the battery thermalmanagement system 54.

As shown in FIG. 3, the control strategy 200 starts at block 202. Atblock 204, a temperature of the battery assembly 24 is monitored. Next,a block 206, the control strategy 200 determines whether the temperatureof the battery assembly 24 is lower than a first temperature threshold.The first temperature threshold may be set at any temperature andrepresents the temperature below which the battery assembly 24 must beheated to ensure proper operation. If the temperature of the batteryassembly 24 is below the first temperature threshold, the controlstrategy 200 may proceed to block 208 by commanding heating of thebattery assembly 24. In one non-limiting embodiment, the batteryassembly 24 is heated by opening the inlet 80 of the three-way valve 62to direct the portion C2 of the coolant C through the passage 77 of thebattery assembly 24. The portion C2 of the coolant C is communicatedfrom the engine 14 and therefore includes a temperature sufficient towarm the battery assembly 24.

Alternatively, if the temperature of the battery assembly 24 is notbelow the first temperature threshold at block 206, the control strategy200 determines whether the temperature of the battery assembly 24 isabove a second temperature threshold at block 210. The secondtemperature threshold is a different threshold value than the firsttemperature threshold, may be set at any temperature, and represents thetemperature above which the battery assembly 24 must be cooled to ensureproper operation.

The control strategy 200 commands cooling of the battery assembly 24 atblock 212 if the temperature of the battery assembly 24 exceeds thesecond temperature threshold. In one non-limiting embodiment, thebattery assembly 24 is cooled by opening the inlet 82 of the three-wayvalve 62 to direct the portion C3 of the coolant C through the passage77 of the battery assembly 24. The portion C3 of the coolant C iscommunicated from the chiller 72 of the chiller loop 70 and thereforeincludes a temperature sufficient to cool the battery assembly 24.

The control strategy 200 returns to block 204 and continues to monitorthe temperature of the battery assembly 24 if it is determined at block210 that the temperature of the battery assembly 24 is not above thesecond temperature threshold.

Although the different non-limiting embodiments are illustrated ashaving specific components or steps, the embodiments of this disclosureare not limited to those particular combinations. It is possible to usesome of the components or features from any of the non-limitingembodiments in combination with features or components from any of theother non-limiting embodiments.

It should be understood that like reference numerals identifycorresponding or similar elements throughout the several drawings. Itshould be understood that although a particular component arrangement isdisclosed and illustrated in these exemplary embodiments, otherarrangements could also benefit from the teachings of this disclosure.

The foregoing description shall be interpreted as illustrative and notin any limiting sense. A worker of ordinary skill in the art wouldunderstand that certain modifications could come within the scope ofthis disclosure. For these reasons, the following claims should bestudied to determine the true scope and content of this disclosure.

What is claimed is:
 1. A method, comprising: monitoring a temperatureassociated with a battery assembly; opening a first inlet of a three-wayvalve to deliver a first portion of a coolant to the battery assemblywhen the temperature is below a first temperature threshold; and openinga second inlet of the three-way valve to deliver a second portion of thecoolant to the battery assembly when the temperature is above a secondtemperature threshold, wherein the first portion of the coolant includesa different temperature than the second portion of the coolant.
 2. Themethod as recited in claim 1, comprising performing a heat transferbetween the coolant and a refrigerant of a refrigerant subsystem.
 3. Themethod as recited in claim 1, wherein opening the first inlet of thethree-way valve communicates the first portion of the coolant from anengine to the battery assembly.
 4. The method as recited in claim 3,wherein opening the second inlet of the three-way valve communicates thesecond portion of the coolant from a chiller to the battery assembly. 5.The method as recited in claim 3, comprising dividing a flow of thefirst portion of the coolant from the engine between a radiator and thethree-way valve.
 6. The method as recited in claim 1, wherein thebattery assembly and the three-way valve are part of a coolant subsystemof a battery thermal management system.
 7. The method as recited inclaim 6, wherein the coolant subsystem further includes an engine, aradiator configured to coolant the engine with the coolant, and aT-joint.
 8. The method as recited in claim 7, wherein the T-joint ispositioned between an outlet of the battery assembly and an inlet of theengine.
 9. The method as recited in claim 7, wherein the T-joint isconfigured to split the coolant that exits from the battery assemblybetween the engine and a chiller.
 10. The method as recited in claim 9,wherein the first portion of the coolant is communicated from the engineand the second portion of the coolant is communicated from the chiller.11. The method as recited in claim 9, wherein the chiller is part of achiller loop of the coolant subsystem.
 12. The method as recited inclaim 7, wherein the battery assembly, the engine, the three-way valve,and the T-joint are positioned in a common coolant line of the coolantsubsystem.
 13. The method as recited in claim 1, wherein the three-wayvalve is positioned immediately upstream from an inlet of the batteryassembly.
 14. The method as recited in claim 1, wherein a temperaturesensor is positioned immediately downstream from an outlet of thebattery assembly.
 15. The method as recited in claim 1, wherein thefirst portion of the coolant is communicated from an engine and thesecond portion of the coolant is communicated from a chiller.
 16. Themethod as recited in claim 1, wherein the first temperature threshold isset at a lower temperature than the second temperature threshold. 17.The method as recited in claim 1, comprising continuing to monitor thetemperature of the battery assembly when the temperature is neither lessthan the first temperature threshold nor greater than the secondtemperature threshold.